This invention relates to a digital frequency measuring device for measuring an input signal frequency in the digital manner.
In ordinary digital frequency measuring devices, the signal to be measured is converted into pulses, and applied to a gate for counting the number of pulses passing therethrough for a certain period of time, and then the count value is displayed in a digital form. With such a method, however, since the signal to be measured and the gate signal are not synchronized with each other, an error appears in the neighborhood of each of the leading and trailing edges of the gate signal. Especially, as the gate time approaches the pulse interval of the signal to be measured, the error increases. For example, in the case of measuring a 1 Hz signal using a gate time of 1 second, an error of .+-.1 Hz appears in the measurement.
Then, the prior art employs the following method to provide for enhanced resolving power. That is, a gate signal synchronized with the signal to be measured is applied to two gate circuits to control them, by which the signal to be measured and the reference clock signal are respectively controlled to obtain gate outputs. The gate outputs are respectively counted by two counters. Then, an integral value of the frequency to be measured is obtained from the count value of the signal to be measured, and values corresponding to errors between the count value of the clock signal and clock pulses appearing at the leading and trailing edges of the gate signal are obtained. And then an operation is performed based on the abovesaid values to obtain a fraction value smaller than one period of the signal to be measured. Since this system performs a numerical operation, it is referred to as the computing counter system, and known to be capable of highly accurate frequency measurement. However, this system requires two counters and a divider, too, and hence has the disadvantage that the construction is relatively complicated as a whole.
One object of this invention is to provide a digital frequency measuring device which is relatively simple in construction and inexpensive but has substantially the same performance as the conventional computing counter system.
Another object of this invention is to provide a digital frequency measuring device which is capable of achieving highly accurate frequency measurement in a short time.
Another object of this invention is to provide a digital frequency measuring device which is capable of achieving highly accurate frequency measurement up to a high frequency region in a short time.
Still another object of this invention is to provide a digital frequency measuring device with which it is also possible to measure time intervals with high resolving power.